Answer:
The average acceleration of train is

Explanation:
We have,
A train that slows down from 15 m/s to 8.6 m/s in 1.2 s. It means that 15 m/s is its initial velocity and 8.6 m/s is its final velocity.
It is required to find the average acceleration of the train.

The average acceleration of train is
. Negative signs shows that the train is decelerating.